1. Field of the Invention
The present invention is generally related to communication systems and in particular to wireless communication systems.
2. Description of the Related Art
To improve the efficiency and throughput of a communication system, a technique known as ARQ (Automatic Repeat reQuest) is often used. The throughput of a system is the amount of source information successfully received by the system (i.e., no errors in the received information) over a defined period of time. ARQ is a procedure used in single antenna systems whereby once a transmitter has transmitted a block of information, it waits for a confirmation message from the receiver informing the transmitter whether the block of information was properly received or was received in error. If the block of information was properly received, the receiver transmits back an ACKnowledge message (ACK); in such a case the transmitter proceeds to the transmission of new information. However, if the block of information was received with errors or received in an unacceptable state (as defined by the system), the receiver transmits back to the transmitter a Negative ACKnowledgement (NACK) message. When the transmitter receives a NACK in response to a transmission, the transmitter can choose to retransmit the same block of information. In particular, the transmitter can retransmit the same block of information a number of times until it receives an ACK. The particular number of retransmissions for any block of information can be defined by the system.
Many communication systems use a particular version of ARQ called Hybrid ARQ or H-ARQ. In H-ARQ, after receiving the transmitted information and prior to transmitting back an ACK or a NACK confirmation message to the transmitter, the receiver performs the following: first, it detects whether the received information contains errors. If the received information contains errors, the receiver attempts to correct the error or errors through well-known error correction techniques. The correction of errors in received information by a receiver is called Forward Error Correction (FEC). There are well known protocols for transmitting and receiving equipment that define how the transmitted information is to be formatted and channel coded prior to transmission. The format is how the information is arranged and the channel coding is making the information more robust (i.e., strategically adding repetitions to the information and/or other kind of redundancies using well-known error correction techniques) so that it is less susceptible to anomalies in the communication channel through which it propagates. If the receiver is able to correct the errors it sends an ACK, but if the receiver is unable to correct the errors it sends a NACK confirmation message to the transmitter, which will retransmit the information. Unlike the ARQ technique, in H-ARQ, each retransmission may be combined with the previous transmissions before being decoded. This “combining” (often referred to as Incremental Redundancy) could improve the useful energy and/or the robustness of the received data.
In order to increase a capacity of a wireless communication system, many wireless communication systems now use multiple antenna systems to transmit and/or receive information. The capacity of a system is the total amount of information conveyed (i.e., transmitted and/or received) by a communication system over a defined period of time. The multiple antenna systems used are typically Multiple Input Multiple Output (MIMO) antenna systems that transmit and receive different information simultaneously. Because of the desirability to use MIMO antenna systems in wireless communication systems, there is also a desire to improve the performance and efficiency of such systems by applying the H-ARQ technique to such systems. However, using H-ARQ in MIMO systems presents several challenges. First, the complexity of managing such a system significantly increases if each antenna performs its own specific H-ARQ procedure. Also if each sub-block of information that is transmitted on each separate antenna is coded differently, the amount of signaling information (information transmitted over a signaling channel used to initiate, maintain and terminate communications) required for such a system would increase significantly as these signaling information are needed for each transmit antenna. A coded sub-block henceforth is the packet sent in each antenna that has been channel coded e.g., an FEC coder. Correspondingly, a coded block refers to the packet generated by the channel coder at the transmitter. The composite of all coded sub-blocks is equal to the coded block. Second, the quality of the channels associated with each antenna varies with time requiring that the antenna transmit and/or receive information at different rates and at different sub-block sizes depending on its state. For multiple H-ARQ processes, each process is typically controlled by a processor thereby tending to increase the complexity of the hardware and software associated with such multiple processes. Increased number of HARQ processes has also been shown to increase the reception time of the packets at the destination.
In a single input single output antenna system operating H-ARQ, the same information is retransmitted on all transmissions of an incoming packet. However, in a multiple input multiple output antennas system, the additional flexibility provided by the multiple antennas can be exploited. As the quality of the channels associated with the antennas changes, the same antenna may not be capable of handling the same information because of its changed state; less of the original information may have to be retransmitted because the antenna's state may be such that it cannot transmit the entire block of the original information. What is therefore needed is a system and method for using one H-ARQ process in a communication system that has a Multiple Input Multiple Output antenna system while avoiding the aforementioned disadvantages.